As a mechanical torque detection mechanism for torque wrenches, there has been suggested a cam-type torque detection mechanism with a cam and a cam roller that is brought into resilient contact with the cam by a spring (Patent Document 1).
FIG. 3(a) shows a torque wrench with the aforementioned conventional cam-type torque detection mechanism. The torque wrench shown in FIG. 3(a) has a tubular head 52 fixed to the front end of a tubular handle 51, and a drive spindle 53 which is rotatably mounted in the head 52 allowing a socket (not shown) or the like to be replaceably mounted thereon. Furthermore, the head 52 is provided therein with a tubular cam 54 which is concentric with the center axis of the drive spindle 53 and rotatably mounted on the outer circumference of the drive spindle 53 with a gap in between. The tubular cam 54 has ratchet teeth 55 on its inner-circumferential surface and four cam portions 56 on its outer circumferential portion along the circumference. Furthermore, the drive spindle 53 is provided, on its outer circumference, with engaging pawls 57 for engagement with the ratchet teeth 55 to serve as a ratchet mechanism. The ratchet mechanism allows the engaging pawls 57 to engage with the ratchet teeth 55 by springs 58, so that the engaging pawls 57 engage with the ratchet teeth 55 when the tubular cam 54 is rotated in the clockwise direction. This makes it possible to integrate the tubular cam 54 with the drive spindle 53, allowing the applied force on the handle 51 to tighten a bolt (not shown) or the like. Note that the ratchet mechanism allows the tubular cam 54 to freely rotate in the counterclockwise direction.
In the handle 51, there is provided a pressing spring 59 for setting torque values and pressing a thrust member 60 disposed at the front end of the handle 51 towards the center axis of the head 52. Between the thrust member 60 and the outer circumferential surface of the tubular cam 54, there is disposed a roller 61 which is rotatable about its own axis. Furthermore, the roller 61 is made freely movable in the longitudinal direction or along the axis L0, so that the spring-actuated thrust member 60 exerts a thrust force to push the roller 61 against the cam portion 56.
In the torque wrench configured as such, suppose that an operational force is applied to the handle 51 in the clockwise direction to tighten a bolt. In this case, if the reactive force exerted on the roller 61 from the contact surface of the cam portion 56 is less than the thrust force received by the roller 61 from the pressing spring 59, then the tubular cam 54 rotates integrally with the handle 51 thereby turning the drive spindle 53. Then, the reactive force increases as the tightening torque increases. When the roller 61 is pushed back towards the cam top of the cam surface against the spring force of the pressing spring 59, the roller 61 reaches a torque peak position P to detect a torque (it is detected that the torque has reached the set value).
As shown in FIG. 3(b), the cam portion 56 or a curved cam surface is made up of: a roller static engagement cam surface 56a of an abrupt slope having a curved surface of radius R0 that agrees with the curved surface of the roller 61 (radius R0); a minus torque cam surface 56b of a gradual slope having a curved surface of radius R2; and a cam top surface 56c with a curved surface of radius R1 formed in between, disposed continuously to or from the roller static engagement surface 56a. Here, the connecting point between the roller static engagement cam surface 56a and the cam top surface 56c is the torque peak position P.